1. Field of the Invention
The present invention relates to a wireless communications system, a base station, and a mobile station, wherein the mobile station performs wireless communication with the base station with handover being smoothly executed.
2. Description of the Related Art
At the present time, a variety of wireless communications systems are being put into practical use. One such wireless communications system is the W-CDMA (Wideband-Code Division Multiple Access) system. Further, the HSDPA (High Speed Downlink Packet Access) using W-CDMA to raise the transmission rate in the downlink by a maximum of 14 Mbps. An adaptive modulation and coding scheme is used in HSDPA systems wherein the system adaptively switches, for example, between a QPSK (Quadrature Phase Shift Keying) modulation scheme and a 16-value QAM (Quadrature Amplitude Modulation) scheme so as to obtain the transmission rate corresponding to the state of the wireless transmission line.
In the HSDPA system, an H-ARQ (Hybrid Automatic Repeat Request) mode is employed. When a mobile station detects an error in reception data from a base station, it requests retransmission of data from the base station. Then the base station retransmits the data and the mobile station performs error correction and decoding processing using both the previously received data and the retransmitted data (refer to, for example, 3GPP TS 25.211 v5.5.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD)) or 3GPP TS 25.212 v5.9.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD)).
Further, the main wireless channels in the HSDPA system include an HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), HS-DPCCH (High-Speed-Dedicated Physical Control Channel), etc.
The HS-SCCH and HS-PDSCH channels explained above are shared channels of the down side (downlink) from a base station to a mobile station in a wireless communications system, and the HS-SCCH is a control channel for transmitting a variety of parameters concerning data transmitted by the HS-PDSCH and a channel for pre-announcement of data transmission. Such parameters include, for example, modulation type information indicating the modulation scheme for transmitting the data by the HS-PDSCH, the number of spreading codes, and the pattern information of rate matching processing applied to the transmission data.
Further, the HS-DPCCH is an individual control channel at the up side (uplink) from a mobile station to a base station in a wireless communications system and is used when transmitting an ACK signal or a NACK signal corresponding to whether normal reception of the data received by the HS-PDSCH is possible or not from the mobile station to the base station. For example, in a case a CRC error is detected in the reception data, etc., the mobile station transmits a NACK signal to the base station. The base station will then perform processing for retransmission based on the NACK signal. Further, the HS-DPCCH is used to report the reception quality (for example SIR (Signal to Interference Ratio)) of the received signal from the base station and periodically transmits the result as a CQI (Channel Quality Indicator) to the base station. The base station judges the quality of the wireless environment in the downlink based on this CQI, switches the modulation scheme to the modulation scheme making higher speed transmission of data possible when the wireless environment is good, or, conversely, switches the modulation scheme to the modulation scheme for transmitting the data at a lower speed when the wireless environment is poor.
FIG. 9 is an explanatory view of the channels in the HSDPA and shows schematic views of a CPICH, P-CCPCH, HS-SCCH, HS-PDSCH, and HS-DPCCH. The CPICH (Common Pilot Channel) and P-CCPCH (Primary Common Control Physical Channel) are common channels in the downlink, wherein the CPICH is a channel utilized for channel estimation, a cell search, and as a timing standard of the other downlink physical channels in the same cell in the mobile station and as a channel for transmitting a so-called pilot signal. Further, the P-CCPCH is a channel for transmitting broadcast information. Further, HS-SCCH, HS-PDSCH, and HS-DPCCH show the above wireless channels and transmit the above CQI and ACK/NACK by HS-DPCCH.
Further, since 15 slots comprise 1 frame, and CPICH is used as the timing standard, the frame heads of P-CCPCH and HS-SCCH coincide with the frame head of the CPICH, but the frame head of HS-PDSCH is delayed by the amount of 2 slots. This allows the mobile station to receive information required to demodulate and discriminate the HS-PDSCH ahead of time. Hence, this information is used in the demodulation and decoding of the HS-PDSCH by notifying, in advance, information regarding the modulation scheme, spreading codes, etc., for pre-announcement by the HS-SCCH. Further, in the HS-SCCH and HS-PDSCH, 3 slots comprise 1 sub frame.
Referring to 3GPP, TS25.212v.5.7.0, the information by the HS-SCCH is described below in (a) to (g).                (a) Xccs (Channelization Code Set Information); 7 bits; Information of spreading codes used in HS-DSCH.        (b) Xms (Modulation Scheme Information); 1 bit; Modulation scheme used in HS-DSCH.        (c) Xtbs (Transport-Block Size Information); 6 bits; Block size of error corrected and encoded transmission data.        (d) Xhap (Hybrid-ARQ Process Information); 3 bits; Retransmission controlled process number.        (e) Xry (Redundancy and Constellation Version); 3 bits; Parameter of rate matching.        (f) Xnd (New Data Indicator); 1 bit; Information of whether is it new data.        (g) Xue (UE Identity); 16 bits; User identification information.        
As described above, the HS-SCCH has a structure of 37 bits in total. By receiving the HS-SCCH, parameters used in the HS-DSCH, such as, the modulation scheme, spreading codes, and error correction can be learned. Accordingly, the HS-PDSCH can be demodulated and decoded according to these parameters.
Xccs described above in (a) indicates the spreading codes when the data is transmitted by the HS-PDSCH and can indicate, for example, a combination of the number of multi-codes and code offset. Further, Xms described above in (b) indicates whether the modulation scheme is, for example, QPSK or 16 QAM by a “0” or “1”. Further, Xtbs described above in (c) indicates data for calculating the size of data transmitted by 1 sub frame of the HS-PDSCH, and Xhap described above in (d) indicates the process number of H-ARQ and uses the same number as the process number of the transmission data of the previous time at the time of retransmission.
Further, Xry described above in (e) indicates the redundancy version parameter and the constellation parameter at the time of retransmission of the HS-PDSCH and includes a case where the parameters are updated and a case where the parameters are not changed in new transmission and retransmission. Further, Xnd described above in (f) is data indicating whether the transmission block of the HS-PDSCH is a new block or a retransmission block. A new block alternately changes the data between “1” and “0”, while a retransmission block keeps the data the same as before, i.e., does not change the data. This enables the two to be differentiated. Further, Xue described above in (g) is identification (ID) information of the mobile station (user).
By receiving the HS-SCCH, the parameters of the modulation scheme, spreading codes, and error correction applied in the HS-PDSCH are recognized, and demodulation and decoding of the HS-PDSCH can be carried out (refer to 3GPP TS 25.211 v5.5.0, 3GPP TS 25.212 v5.9.0, 3GPP TS 25.214 v5.7.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD)).
A simple explanation will be given of the operation of packet transmission applying the HSDPA system to the above W-CDMA system.
The base station transmits the data via the shared channel (HS-PDSCH) and selects the mobile station to which data is to be transmitted via this shared channel from among a plurality of mobile stations. Then, the base station sequentially transmits the radio frames storing the data addressed to the selected mobile station. At this time, the base station pre-announces the transmission of data prior to data transmission via the control channel (HS-SCCH).
For example, as shown in FIG. 10, the base station transmits signaling by the control channel (HS-SCCH) indicating transmission of data (packets) when transmitting data (packets) to a certain mobile station. On the other hand, the mobile station constantly monitors this control channel and performs processing for reception of the data packets by detecting signaling addressed to it.
It should be noted that, in FIG. 10, the signaling signals, indicated by broken lines, indicate signals addressed to other mobile stations, the case where the mobile station for the pre-announcement of transmission does not exist, or the case where a signaling signal is not to be transmitted. Naturally data for a portion in which the signaling signal is not transmitted is not transmitted.
As explained above, when transmitting data from the base station via the shared channel and performing handover processing, the mobile station must measure the received signal from a peripheral cell. For such a handover, the following technique has been proposed.
For example, in a wireless communications system asynchronously performing packet communication, the mobile station receives a beacon transmitted from a base station at a constant cycle, measures the level of reception and, when the measurement result is less than a threshold value, transmits a state transition packet to the base station and shifts to a communication suppression mode such as a power save mode. A wireless communications system has been proposed in which the mobile station stops transmitting and receiving data in this communication suppression mode, searches for another base station during this communication suppression mode and, when shifting from this communication suppression mode to the normal mode, transmits a state transition packet and returns to the normal mode (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2003-158481).
Further, in a wireless communications system using the TDMA system, a wireless communications system has been proposed which detects the field strength of a transmission signal from a peripheral cell by utilizing the time of idle time slots originally not used by the mobile station and transmits the detection result to a base station to request handover (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 3-268697).
In order for a mobile station to secure handover in a wireless communications system, it therefore has to detect a peripheral cell, but provisioning a receiving unit merely for the detection of a peripheral cell is impractical/impossible, both in terms of space and in terms of power consumption. Accordingly, the mobile station detects a peripheral cell using the frequency of the peripheral cell, the wireless access scheme, etc. However, in the HSDPA and next generation mobile communications, signals are transmitted and received by the packet format through a shared channel. Therefore, there are cases where signals are transmitted in frames of consecutive packets and cases where signals are intermittently transmitted. Whether or not such packets are transmitted is notified for each frame by the signaling from the base station. Therefore, a mobile station must constantly receive the signal from the base station. This makes it difficult to detect the peripheral cell efficiently.
Further, in the conventional example where a mobile station receives a beacon and shifts to a communication suppression mode when the result of measurement of the level of reception is less than a threshold value, there is a problem in that a period where no data can be received at all is continuously generated for a predetermined time during this communication suppression mode.
Namely, this presents a problem in that the transmission efficiency is remarkably lowered. Further, in order to store data in this communication suppression mode, it is necessary to provide a buffer memory having a relatively large storage capacity.